North Korea has triggered global alarm with its most powerful atomic test, claiming it was a hydrogen bomb that could be mounted onto a long-range missile.

South Korea's Capital Defense Command soldiers take part in a military drill in Seoul, following North Korea's sixth nuclear test.(AFP)

South Korea’s navy held major live-fire drills on Tuesday to warn the North against any provocations at sea, it said, two days after Pyongyang’s biggest nuclear test to date.

The drills, conducted in the East Sea (Sea of Japan), involved the 2,500-tonne frigate Gangwon, a 1,000-tonne patrol ship and 400-tonne guided-missile vessels, among others, the Navy said in a statement.

“If the enemy launches a provocation above water or under water, we will immediately hit back to bury them at sea,” Captain Choi Young-chan, commander of the 13th Maritime Battle Group, said in a statement.

North Korea on Sunday triggered global alarm with by far its most powerful atomic test to date, claiming it was a hydrogen bomb that could be mounted onto a long-range missile.

On Monday, the South’s military launched a volley of ballistic missiles simulating an attack on the North’s nuclear test site.

US President Donald Trump and South Korea’s leader Moon Jae-In agreed during a phone call late Monday to remove limits on the payload of the South’s missiles, fixed at 500 kg, according to a 2001 bilateral agreement.

Trump also said he was willing to approve the sale of “many billions of dollars’ worth of military weapons and equipment from the United States by South Korea”, according to a statement released by the White House.

Tensions have mounted on the Korean peninsula following a series of missile launches by the North, including two intercontinental ballistic missiles (ICBMs) that apparently brought much of the US mainland into range.